In a cutting insert formed into a circular shape in a planar view, that is, in a so-called a circular (round) cutting insert, a top surface as a rake face and a bottom surface formed at a position opposite to the top surface are formed into circular shapes, a side surface connecting the top surface and the bottom surface to each other is formed into an approximately columnar shape or an approximately conical shape, and cutting edges are provided in some or all of ridgeline at which the side surface intersects with the top surface. In addition, in a double-sided cutting insert formed in a circular shape, cutting edges are also provided on ridgeline at which the bottom surface and the side surface intersect with each other.
In a case where the cutting insert formed into a circular shape (hereinafter, referred to as a “circular cutting insert”) is attached to an insert mounting seat provided on a tool main body of the indexable rotary cutting tool and is fixed to the insert mounting seat by tightening a clamp screw, and a cutting operation of a workpiece is performed, there are the following tasks (1) to (3), and in the related art, solutions with respect to the tasks are suggested.
Task (1)
In the circular cutting insert, cutting edges formed into a circular shape are provided on the ridgeline at which the top surface intersects with the side surface or the ridgelines at which the top surface and the bottom surface intersect with the side surface, and thus, it is necessary to effectively use the regions of the cutting edges. Accordingly, whenever the circular cutting insert is reattached to the insert mounting seat of the tool main body, it is necessary to accurately position the circular cutting insert so as to fix the circular cutting insert to the tool main body by tightening the clamp screw (an indexing function is required). Particularly, when a portion (cutting edge portion) of the cutting edges used during the cutting operation wears out, in order to use an unused portion during the cutting operation the next time, the circular cutting insert is rotated about a center of a circle of the cutting edges by a predetermined angle. Accordingly, in a state where the positions of the cutting edges are shifted, the circular cutting insert is reattached to the insert mounting seat. In the reattachment of the cutting edges, it is necessary to attach the circular cutting insert to the insert mounting seat in a state where the unused cutting edges are accurately positioned without erroneous operations.
Task (2)
Even when the circular cutting insert is firmly fixed to the insert mounting seat, the cutting insert is easily displaced by a cutting force loaded to the cutting insert during a cutting operation of a workpiece. The reason for this is that the circular cutting insert is fixed to the insert mounting seat of the tool main body by tightening the clamp screw; however, the side surface of the circular cutting insert is formed into a columnar shape (cylindrical shape) or a conical shape, and thus, a reaction force with respect to the cutting force during the cutting operation acts as a force trying to rotate the circular cutting insert about the clamp screw (a turning force about the clamp screw). If the circular cutting insert fixed to the insert mounting seat is rotated by the turning force (rotational moment) and the fixed position thereof is slightly shifted, machining accuracy of a machined surface of the workpiece deteriorates, vibrations are generated, abnormal wear of the cutting edges occurs, and thus, the cutting edges are damaged. Accordingly, in the indexable rotary cutting tool to which the circular cutting insert is attached, a function for preventing turning (or rotation) of the circular cutting insert during the cutting operation (antirotation function) is required.
Task (3)
In general, when a workpiece is cut using the indexable rotary cutting tool to which the cutting insert is attached, the cutting force acting on the cutting insert is applied in a direction of pressing the cutting edge portion (a portion of the cutting edge used for cutting) from the top surface to the insert mounting seat of the indexable rotary cutting tool. In addition, in the cutting insert, a stress acting on the cutting edge portion and a repulsive force with respect to the stress are generated. During a cutting operation in a normal state, the stress acting on the cutting edge portion and the repulsive force with respect to the stress mainly act on the clamp screw by which the cutting insert is fixed to the tool main body.
However, in a transient state during the cutting operation, if the stress acting on the cutting edge portion and the repulsive force with respect to the stress are not balanced with each other, vibrations may occur. The transient state during the cutting operation indicates a moment when the cutting edge bites into the workpiece, or the like. The clamp screw is loosened or the installation (attachment) position of the cutting insert with respect to the insert mounting seat is displaced by the vibrations, and thus, a so-called displacement of the cutting insert may occur. If the vibrations and the displacement of the cutting insert occur, the cutting edge is damaged, and there are problems that favorable machining accuracy and improved machined surface roughness with respect to the workpiece cannot be obtained. In addition, there is a disadvantage that noise occurs during the cutting operation.
Moreover, in recent years, in order to improve machining efficiency with respect to a workpiece, a high-speed cutting operation is required. If the high-speed cutting operation is performed, when the cutting edge of the cutting insert bites into a workpiece, vibrations are likely to be generated by the impact. Accordingly, in order to realize the high-speed cutting operation in the indexable rotary cutting tool to which the circular cutting insert is attached, it is very important to provide a function for preventing or suppressing occurrence of the vibrations (antivibration function).
In the indexable rotary cutting tool to which the circular cutting insert is attached, among the above-described tasks (1) to (3), an improvement plan for the tasks (1) and (2) are proposed by many patent applications in the related art. However, there are few suggestions with respect to an improvement plan for the task (3) in addition the tasks (1) and (2). The reason for this is that a favorable improvement plan for the tasks (1) and (2) is based on a technical idea that vibrations are not generated during the cutting operation.
For example, the improvement plans for the tasks (1) and 2 are suggested by PTL 1 to PTL 3 below.
PTL 1 (Japanese Unexamined Patent Application, First Publication No. 2011-245585) suggests that in a positive round cutting insert, the cutting insert for increasing reliability of fixing with respect to an insert mounting seat and a cutting tool using the cutting insert. In the invention described in PTL 1, a plurality of planar indexing surfaces of which inclination angles are larger than the inclination angle of a side surface of the cutting insert are provided on the side surface at constant pitches in a circumferential direction, a vertically long protrusion is provided at a center portion of each of the indexing surfaces in the circumferential direction, the protrusion is inserted to engage with a recessed portion provided on a seating side surface (wall surface) of an insert mounting seat of a tool main body, and in this state, the indexing surfaces positioned at different positions abut on a plurality of seating side surfaces. Accordingly, the cutting insert is positioned on the insert mounting seat, and rotation of the cutting insert is prevented. In addition, PTL 1 describes that in the cutting tool, an indexing number of the cutting edges of the cutting insert is set to four, two seating side surfaces whose directions are different from each other by 90 degrees are formed on the insert mounting seat of the tool main body, and the adjacent indexing surfaces of the cutting insert are restrained by two seating site surfaces.
PTL 2 (Published Japanese Translation No. 2012-525268 of the PCT International Publication) suggests a double-sided cutting insert formed into a circular shape and a cutting tool using the cutting insert. The cutting insert described in PTL 2 has a configuration in which a plurality of antirotation surfaces substantially perpendicular to a top surface and a bottom surface are provided in a circumferential direction of a side surface of the cutting insert. Meanwhile, protrusion surfaces which are aligned with the antirotation surface of the cutting insert and prevent the rotation of the cutting insert, are provided on a pocket side surface (a wall surface provided on the insert mounting seat) of the tool main body. Accordingly, a wide attachment area is secured between the cutting insert and the cutting tool to prevent the rotation of the cutting insert.
PTL 3 (Specification of U.S. Pat. No. 6,607,335) suggests an invention which relates to a cutting insert attached to an indexable rotary cutting tool and the cutting tool. The cutting tool described in PTL 3 does not adopt the indexing means and the antirotation means of the cutting insert described in PTLs 1 and 2. That is, the means of restraining the rotation restraining faces formed on the side surface of the cutting insert or the protrusions of the rotation restraining faces, is not provided on a restraining wall surface erected to the seating surface provided on the insert mounting seat of the tool main body. However, FIG. 14 of PTL 3 shows an embodiment of a cutting insert, in which the center of the side surface of the double-sided circular cutting insert is a ridgeline at which the side surface and a base surface (intermediate plane M) of the cutting insert intersect with each other, and the side surface each between the ridgeline and the top surface and between the ridgeline and the bottom surface includes eight planes sequentially connected along the ridgeline to have an octagonal cross-section.
PTL 4 and PTL 5 suggest inventions relating to a cutting tool having a means of preventing occurrence of vibrations in the indexable rotary cutting tool to which the circular cutting insert is attached.
PTL 4 (Japanese Unexamined Patent Application, First Publication No. 2012-206249) suggests an invention relating to a double-sided circular cutting insert for preventing occurrence of vibrations caused by loosening of a tightening screw in an antirotation mechanism which can be easily and inexpensively manufactured. The circular cutting insert suggested in PTL 4 includes a plurality of dimples (recessed portions) formed on the top surface and a plurality of other dimples formed on the bottom surface facing the top surface to be capable of an indexing. The cutting insert is removably accommodated in an insert accommodation pocket of the tool main body, and the insert accommodation pocket includes a single protrusion which can be accommodated in one of the plurality of dimples. Therefore, according to cooperation of the protrusion of the insert accommodation pocket and one of a plurality of dimples of the cutting insert, the indexing of the round cutting insert attached to the insert accommodation pocket can be performed, and the rotation of the cutting insert during the cutting operation is prevented. As the effect thereof, PTL 4 describes that a mechanism for preventing occurrence of vibrations caused by the loosening of the tightening screw is provided.
PTL 5 (Published Japanese Translation No. 2002-527251 of the PCT International Publication) suggests an invention relating to an antirotation attachment mechanism of a circular cutting insert including an antirotation means of the circular cutting insert and an antivibration means thereof. PTL 5 states that the antirotation attachment mechanism includes a plurality of curved stopping surfaces inclined to a cylindrical side surface of the cutting insert around the cylindrical side surface and an antirotation surface which forms an interference fit joint surface engaging with the inclined disposition portion of the curved stopping surface of the cutting insert in one line contact manner on the insert mounting seat (pocket) of the tool main body. In addition, PTL 5 states that the side surface of the pocket includes a semicircular upper portion for coming into direct contact with the upper side wall of the cutting insert.
In addition, Specification of PTL 5 describes that “a supporting contact between an upper portion 60 of a pocket side surface 50 and an upper side wall 64 of an insert 3 is important for two reasons. Firstly, most of lateral vibration loads applied to the insert 3 are absorbed by a wide range of semicircular contact between the side surface 50 of the pocket 5 and the upper portion 60 of the insert side wall 21 during the cutting operation in order to protect the antirotation mechanism 1 from the vibration loads. Secondly, according to this configuration, most of the lateral vibrations concentrate on the strongest portion of the insert 3, that is, the upper side wall 64 which has the largest diameter of the insert and in which a material of an insert main body is little or not removed at all since the curved stopping surfaces 35 are provided”. That is, PTL 5 describes that the supporting contact between the upper portion 60 of the pocket side surface 50 and the upper side wall 64 of the insert 3 absorbs lateral vibrations occurring during the cutting operation.